Three weeks ago, the San Diego Union-Tribune Editorial Board opposed Measure A, which would add a half-cent to sales taxes to pay for infrastructure and transportation projects around San Diego County. We said the San Diego Association of Governments’ proposal was built on outdated assumptions that are likely to be shredded over time by new technologies such as driverless vehicles. A new report by Bloomberg New Energy Finance and McKinsey & Co. says autonomous cars are likely to be so cheap that they sharply undermine mass transit.

Now there’s another reason to question Measure A. A recent analysis by Voice of San Diego detailed how the planning agency’s revenue projections for a sales tax hike approved in 2004 are falling short following the recession and that its assumption that Measure A would generate $18 billion over the next 40 years is based on the same methodology. SANDAG expects steady annual — and cumulatively huge — increases in how much sales tax revenue it gets from the average county resident. That prompted skepticism from finance experts interviewed by Voice of San Diego.

In an interview with an editorial writer, Gary Gallegos, SANDAG’s executive director, defended his agency’s projections as “a bit aggressive but reasonable.” He said SANDAG has hedged against missed projections by leveraging local dollars to get state and federal aid and borrowing at lower than expected rates. He also said that project costs can go down when sales tax revenue projections dip. It’s “a lot of crystal balling,” he said.

In other words, it’s a bit of a gamble, even with SANDAG’s good track record so far. It seems Measure A doesn’t just have questionable assumptions about the future of transportation, it has questionable assumptions about how we’ll pay for that future. We again urge voters to reject Measure A.

To read all the Union-Tribune editorial board’s endorsements and candidate interviews this election season, visit sdut.us/endorsements.